In recent years numerous earth satelites have been placed in earth orbit for both civil and military purposes, e.g., communications, surveillance, scientific investigations, etc. Initially these satelites were placed in orbit by multi-stage conventional liquid or solid fuel rockets, the upper stage of which functioned to transfer the payload from an initial orbit achieved by the first stages of the rocket into another orbit of different shape or distance from the earth. These final stage or orbit "transfer" stages have achieved high states of development and reliability. However, significant economic and other practical problems are encountered in the use of such present technology because of the primary fact that each transfer vehicle can only accommodate payloads which are within a fairly narrow weight-size-shape range. Consequently, users of the currently-available range of space technology are constantly faced with the necessity of developing satelite transfer vehicles specifically adapted for orbit placement of payloads when the size or weight (and sometimes the shape) of the payload is changed. This results in multiplication of development and engineering costs, higher production costs and lengthy delays.
More recently, the United States of America has developed and demonstrated the technical and commercial viability of the so-called space shuttle (more officially designated the Space Transportation System or "STS"). This well-known earth-launch vehicle provides, for the first time, a reusable, highly versatile earth-launch which can carry a wide variety of payloads into LEO and which can then be deployed from the cargo bay of the STS and re-launched or "transferred" from LEO to another orbit utilizing a separate vehicle carried with the payloads in the cargo bay to accomplish the transfer. The most familiar of such transfer vehicles are the so-called "PAM" (payload assist module) vehicles which were adapted for use with the STS from vehicles which formerly served this function in connection with unmanned "Delta" rockets. However, although PAM vehicles have been successfully employed in conjunction with transferring payloads from the LEO of the STS into GEO, there are still significant practical economic limitations on their use with certain current and technologically predictable payloads which are both larger (heavier) than and smaller (lighter) than the payloads for which the modified PAMs were developed.
Thus, in general, according to the current approach for transferring a satelite or other payload from an earth-launch vehicle in one trajectory to another trajectory, the major orbital maneuvers are provided by separate systems, i.e., a separate "upper stage" propulsion vehicle to transfer the spacecraft to apogee and the payload spacecraft which is provided with its own propulsion system for apogee maneuvers to insert the spacecraft into an operational orbit after it has been transferred from the low-earth parking orbit of the earth-launch vehicle. Typically, the upper stage vehicle is expendable and furnished as a separate unit from the spacecraft and must be integrated with the spacecraft to form the "payload" of the earth-launch vehicle.